IMAGE  EVALUATION 
TEST  TARGET  (MT-3) 


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Carparation 


at  Vm|T  %UUN  STRWT 

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(7U)I72-4S0I 


CIHM/ICMH 

Microfiche 

Series. 


CIHM/iCIVIH 
Collection  de 
microfiches. 


Canadian  Inttituta  for  Historical  IMicroroproductiona  /  Inatitut  Canadian  da  microraproductiona  historiquaa 


Tachnical  and  Bibliographic  Notoa/Notas  tachniquaa  at  bibiiograpliiquaa 


Tha  Inatituta  haa  attamptad  to  obtain  tha  baat 
original  copy  availabia  for  filming.  Faaturaa  of  thia 
copy  which  may  ba  bibliographically  uniqua, 
wliich  may  altar  any  of  tha  imagaa  in  tha 
raproduction.  or  which  may  aignificantly  changa 
tha  uaual  mathod  of  filming,  ara  chackad  balow. 


D 


D 


D 
D 


D 


D 


Colourad  covara/ 
Couvartura  da  coulaur 


I     I   Covara  damagad/ 


Couvartura  andommagte 

Covara  raatorad  and/or  laminatad/ 
Couvartura  raataurAa  at/ou  palliculAa 


I     I   Covar  titia  miaaing/ 


La  titra  da  couvartura  manqua 

Colourad  mapa/ 

Cartaa  gAographiquaa  an  coulaur 

Colourad  ink  (i.a.  othar  than  blua  or  black)/ 
Encra  da  coulaur  (i.a.  autra  qua  blaua  ou  noira) 


I     I   Colourad  plataa  and/or  illuatratlona/ 


PlancKia  at/ou  illuatratlona  an  coulaur 


Bound  with  othar  matarial/ 
RaM  avac  d'autraa  documanta 


Tight  binding  may  cauaa  ahadowa  or  diatortion 
along  Intarlor  margin/ 

La  r9  liura  aarrte  paut  cauaar  da  i'ombra  ou  da  la 
diatortion  la  long  da  la  marga  intiriaura 

Blank  iaavaa  addad  during  raatoration  may 
appaar  within  tha  taxt.  Whanavar  poaaibia,  thaaa 
hava  baan  omittad  from  filming/ 
II  aa  paut  qua  cartainaa  pagaa  blanchaa  ajoutiaa 
lora  d'una  raatauration  apparaiaaant  dana  la  taxta, 
mala,  loraqua  cala  Atalt  poaaibia.  caa  pagaa  n'ont 
paa  4t«  film4aa. 

Additional  commanta:/ 
Commantairaa  aupplAmantairaa: 


L'Inatitut  a  microfilm*  la  maiilaur  axamplaira 
qu'il  lui  a  At*  poaaibia  da  aa  procurar.  Laa  dAtaiia 
da  cat  axamplaira  qui  aont  paut-Atra  uniquaa  du 
point  da  vua  bibliographiqua,  qui  pauvant  modifiar 
una  imaga  raprodulta,  ou  qui  pauvant  axigar  una 
modification  dana  la  mAthoda  normala  da  fllmaga 
aont  indiquiia  ci-daaaoua. 


|~~1  Colourad  pagaa/ 


D 


Pagaa  da  coulaur 

Pagaa  damagad/ 
Pagaa  andommagtoa 

Pfigaa  raatorad  and/oi 

Pagaa  raataurAaa  at/ou  palliculAaa 

Pagaa  diacolourad,  atalnad  or  foxai 
PagM  dAcolorAaa,  tachattea  ou  piquAaa 

Pagaa  datachad/ 
Pagos  dAtach^aa 

Showthrough> 
Tranaparanca 

Quality  of  prim 

Quality  intgala  da  I'lmpraaaion 

Includaa  aupplamantary  matarli 
Comprand  du  material  auppMmantaira 

Only  aditlon  availabia/ 
Saula  MMon  diaponibia 


Tl 
to 


I — I  Pagaa  damagad/ 

□  Pfigaa  raatorad  and/or  laminatad/ 
Pagaa 

I — I  Pagaa  diacolourad,  atalnad  or  foxad/ 

r~|  Pagaa  datachad/ 

r     I  Showthrough/ 

I     I  Quality  of  print  variaa/ 

I     I  Includaa  aupplamantary  matarial/ 

I — I  Only  aditlon  availabia/ 


Tl 

P< 
o1 
fil 


O 
b4 
tl 
ai 

01 

fil 
ai 

01 


Tl 
al 

Tl 

VM 

M 
dl 
m 
b 
ri 
ri 
n 


Pagaa  wholly  or  partially  obacurad  by  arrrta 
allpa,  tiaauaa,  ate,  hava  baan  rafllmad  to 
anaura  tha  baat  poaaibia  imaga/ 
Laa  pagaa  totalamant  ou  partlallamant 
obacurciaa  par  un  fauillat  d'arrata.  un«  palura, 
ate,  ont  4t4  filmAaa  A  nouvaau  da  fci^on  A 
obtanir  la  maiilaura  Imaga  poaaibia. 


Thia  itam  ia  filma<^  at  tha  raduction  ratio  chaekad  balow/ 

Ca  documant  aat  filmA  au  taux  da  reduction  indlqu*  ci-daaaoua 

10X                          14X                           18X                          »X 

2SX 

aox 

X! 

12X 

16X 

20X 

24X                          2IX                          32X 

tails 
I  du 
odifior 
'  une 
mag* 


Th«  copy  filmad  hara  hat  baan  raproducad  thanks 
to  tha  ganarosity  of: 

Douglas  Library 
Quaan's  Ur?'*varsity 

Tha  imagas  jppaaring  hara  ara  tha  bast  quality 
posslbia  considaring  tha  condition  and  lagibility 
of  tha  original  copy  and  in  kaaping  with  tha 
filming  contract  spacif icationa. 


Original  copies  in  printad  papar  covars  ara  filmad 
beginning  with  tha  front  covar  and  anding  on 
tha  last  paga  with  a  printad  or  lllustratad  intpras- 
sion,  or  tha  back  covar  whan  appropriata.  All 
othar  original  copiaa  ara  filmad  beginning  on  tha 
first  paga  with  a  printad  or  lllustratad  Impraa- 
sion,  and  anding  on  tha  last  paga  with  a  printad 
or  lllustratad  impraaslon. 


Tha  last  racordad  frama  on  aach  microfiche 
shall  contain  the  symbol  -^  (meaning  "CON- 
TINUED"), or  the  symbol  ▼  (meaning  "END"), 
whichever  applies. 

Maps,  plates,  charts,  etc.,  may  be  filmed  at 
different  reduction  ratios.  Those  too  large  to  be 
entirely  included  in  one  exposure  ara  filmed 
beginning  In  the  upper  left  hand  corner,  left  to 
right  and  top  to  bottom,  as  many  frames  as 
required.  The  following  diagrams  illustrate  the 
method: 


L'exemplaira  film4  fut  reproduit  grlce  i  la 
gAnArosIti  da: 

Douglas  Library 
Queen's  University 

Los  imagas  suivantas  ont  4tA  reproduites  avec  le 
plus  grand  soln,  compta  tenu  de  la  condition  at 
de  la  nattet*  de  Texemplaira  fiimA,  et  en 
conformity  avec  las  conditions  du  contrat  de 
filmaga. 

Lea  exemplairas  originaux  dont  la  couverture  en 
papier  eat  imprimta  sent  fllmte  en  commenpant 
par  le  premier  plat  et  en  terminant  soit  par  la 
darnlAre  paga  qui  comporte  une  empreinte 
d'Impreeslon  ou  d'iliustration.  soit  par  ie  second 
plat,  salon  le  caa.  Tous  lee  autres  exemplairas 
originaux  sent  fllmte  en  commen^ant  par  la 
pramlAre  page  qui  comporte  une  empreinte 
d'Impreeslon  ou  d'iliustration  et  en  terminant  par 
la  darnlAre  page  qui  comporte  une  telle 
empreinte. 

Un  dee  symboles  suivants  apparattra  sur  la 
darniire  image  de  cheque  microfiche,  selon  ie 
cas:  ie  symbols  — ►  signifle  "A  SUIVRE",  ie 
symbols  ▼  signifle  "FIN". 

Les  cartes,  planches,  tableaux,  etc.,  peuvent  Atre 
fllmte  k  des  taux  do  rMuction  diffArants. 
Lorsqua  la  document  est  trop  grand  pour  Atre 
reproduit  en  un  seul  clichA,  11  est  film*  A  partir 
da  i'angle  supArieur  geuche,  de  gauche  k  droite, 
et  de  haut  en  baa,  en  prenant  ie  nombre 
d'Images  niceesalre.  Lea  diagrammes  suivants 
lilustrent  la  mithode. 


errrta 
to 


)  pelure, 
on  A 


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32X 


1 

2 

3 

1 

2 

3 

4 

6 

6 

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tDOUQlAS 
LifeRAKy 

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AT  klNQSCON 

kiNQSTON     ONTARIO     CANADA 


llliiiiiJiiidllllill^^tfii 


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li"^':*?!! 


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A  REPLY 


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VsH 


lit 


.../oiijisjiiljivfi/: 


TO 


THE     RECENT     CRITICISM 


1 


Made  by  Mr.   Edward  Wasell, 


•;K,^';.i 


UPON   THE 


'sm 


awiiis 


NIAGARA 


r «  »  ..S^MV^AY  Suspension  Bridge. 


BY 


W.  A.  ROEBLING,  C.  E. 


\   ^     V 


NEW   YORK; 
S.    B.    LEVERICH, 

105   FULTON  ST. 
1877. 


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NiAciARA  Railway  Suspension  Bridge. 


A    REPLY    TO    THE    RECENT    CRITICISM    MADE 
BV    MR.    EDWARD    WASELL. 


Nearly  twenty-two  years  have  now  elapsed  since 
the  completion  of  the  Niagara  Suspension  Bridge. 
During  this  time  scarcely  a  month  has  passed  but  the 
structure  has  been  attacked,  or  criticised  in  a  friendly 
as  well  as  unfriendly  spirit ;  but  generally  from 
motives  not  entirely  disinterested.  One  fault  is 
common  to  all  the  critics,  viz. :  their  judgment  is 
based  on  what  they  would  do  now,  and  not  what 
they  would  have  done  twenty-six  years  ago.*  The.se 
pamphleteers  find  their  echo  in  the  general  press ; 
and  when,  after  a  twelvemonth,  the  round  of  the 
country  has  been  made,  another  one  stands  ready  to 
repeat  the  farce.  Among  the  latest  of  these  pro- 
ductions is  a  modest  little  pamphlet  by  Mr.  Wasell, 


*  In  criticising  work  built  even  so  recently  as  25  years  ago,  it 
should  in  justice  be  remembered  that  the  era  of  iron  chords,  beams, 
angle  ironsv  etc.,  had  not  yet  arrived,  and  hence  wood  had  to  be  used 
in  situations  where  it  would  be  considered  inadmissible  at  the  present 
time. 


"Vn   O. 


■m  -r, 


of  Dijjby,  Nova  Scotia,  late  Assistant   Engineer  on 
the  Great  Western  Railway  of  Canada. 

In  his  solemn  warnings  to  the  travelling  public, 
predicting  their  early  engiilfment  in  the  fearful  chasm, 
he  is  apparently  actuated  by  sentiments  of  the  purest 
hlimanity  ;  but  when  we  turn  to  the  preface  of  his 
pamphlet,  we  find  that  our  author  has  blessed  the 
world  with  still  another  form  of  suspension  truss, 
and  would  particularly  like  to  try  it  near  Oueenstown, 
some  eight  miles  below  the  present  bridge. 

While  mentioning  this  location  I  may  add,  that 
five  years  ago  I  had  several  consultations  with  Mr. 
Reed,  Chief  Engineer  of  the  Great  Western  Road, 
concerning  the  project  of  a  railway  suspension  bridge 
at  that  point,  and  even  went  so  far  as  to  make  two 
estimates  .)f  cost,  which,  however,  proved  too  high. 
It  appeared  to  be  the  object  of  the  Great  Western 
to  form  a  connection  with  the  Ontario  Lake  Shore 
Road  at  this  point.  When  this  was  abandoned,  nego- 
tiations were  commenced  for  the  purchase  of  the 
Niagara  bridge  itself,  but  the  financial  embarassments 
of  the  Company  put  an  end  to  these  schemes. 

With  these  preliminary  remarks,  I  will  pass  to  a 
consideration  of  the  statements  made  by  Mr.  Wasell 
in  his  pamphlet.  The  first  thing  he  gives  (and  upon 
which  all  his  computations  are  based)  is  a  table  of 
quantities.     In  this  he  states  that 


P 


^ 


'IliL-  "roatc^t  rollintr  load  is — on  railwav     - 
"  "  "  "        — on  carriage  way 

Total 


470  tons. 
lOo     " 


630 


Turning  to   Mr.  John  A.  Roebling's  published  state- 


V 


«» 


5 
ments,  I  find  he  wrote  as  follows  |  italics  are  his  own  | : 

'  Trains  of  more  than  2CX)  tons  vveiglit  will  only  cross  the  bridge 
experimentally,  or  at  any  rate  but  very  seldom.  Add  to  this  a  number 
of  teams  and  ,)ersons  on  both  floors,  weij^ing  in  all  about  50  tons, 
and  we  have  a  total  weight  of  250  tons,  to  which  the  bridge  will  be 
oaasionaliy  subjected.    Onlinary  passing  loads  are  within  this  figure." 

• 

The  experimental  train  passed  over  on  March 
i8th,  1855,  was  especially  prepared  to  cover  the  entire 
bridge,  and  was  estimated  to  weigh  326  tons.  This 
train  consisted  of  twenty  fully-loaded  freight  cars, 
pushed  by  a  26-ton  engine.  The  load  of  470  tons 
assumed  by  Mr.  Wasell  as  the  maximum  load  on  the 
railway  track,  is  almost  equal  to  a  train  of  locomo- 
tives, and  is  simply  preposterous.  If  ever  such  a  load 
has  been  taken  across  the  bridge,  it  has  been  done 
surreptitiously,  and  with  a  design  to  injure  it. 

With  regard  to  loads  on  the  carriage-way,  it  is 
difificult  to  say  how  small  they  really  are.  The  erec- 
tion of  the  Clifton  bridge  took  away  nearly  half  the 
travel  on  this  floor  of  the  bridge ;  so  that  it  is  literally 
true  that  the  "  solitary  horseman  "  is  now  the  rule 
instead  of  the  exception,  as  formerly. 

To  establish  his  assumption  on  this  point,  Mr. 
Wasell  finds  it  necessary  to  pack  the  lower  floor  with 
people,  next  to  subject  them  to  a  freezing  temperature 
of  20  degrees  below  zero  ;  and  at  last  to  blow  upon 
them  with  a  wind  of  sufficient  force,  to  hurl  them,  and 
the  locomotive  over  their  heads,  into  the  river  below. 
The  greatest  load  that  was  ever  put  upon  the 
bridge  to  my  knowledge,  was  360  tons."^     Assuming 

*  This  was  when  the  lower  floor  was  packed  full  of  spectators  to 
view  the  spectacle  of  Rlondin  crossing  the  chasm  on  his  rope.  The 
crowding  was  allowed  once  or  twice  only,  and  was  then  stopped. 


I;pf!'!:l|p|[  ■ '  S)!*!  ''ip|!|ilil.1 1 


however  that  Mr.  Wasell  really  believes,  that  the 
bridge  is  liable  to  be  frequently  called  upon  to  sustain 
a  passing  load  of  630  tons,  let  us  see  the  light  in 
which  it  places  him. 

It  is  a  matter  of  history,  that  when  the  bridge 
was  about  half  completed,  it  was  leased  (or  rather 
given  away)  to  the  Great  Western  R.  R.  Co.  This 
was  owing  to  the  want  of  faith  in  the  work,  on  the 
part  of  some  of  the  bridge  directors.  Mr.  Wasell 
states  that  for  a  portion  of  the  time  that  the  lease 
existed,  he  was  in  the  position  of  Chief  Assistant 
Engineer  on  the  Great  Western,  and  therefore  one 
of  its  executive  officers.  Hence  this  overloading 
must  have  been  done  with  the  knowledge  and  con- 
sent of  himself  and  associates ;  and  now  he  dares 
to  charge  the  structure  with  weakness,  which  he 
himself  has  helped  to  produce,  by  loading  it  to 
double  the  test  load,  and  2)/^  times  the  maximmn 
load  which  it  was  built  to  carry.  Fortunately, 
by  a  decision  of  the  courts,  the  railroad  floor 
has  been  relegated  to  the  Bridge  company ;  and 
there  is  now  an  end  to  such 
loadinT. 


outrageous   over- 


As  a  matter  of  fact,  the  portion  of  the  load  that 
has  been  substantially  increased,  is  the  weight  of  the 
locomotives.  This  has  been  gradually  raised  from  25 
tons,  up  to  40  and.  even  45  tons.  The  effect  of  such 
a  concentration  of  load  has  been,  not  to  injure  the 
cables,  but  to  rack  the  trusses  to  pieces.  The  only 
present  defect  of  the  bridge  is  in  the  trusses,  and  it  is 
these  which  require  immediate  attention.  Had  Mr. 
Wasell  directed  his  criticism  against  this  part  of  the 


J 


rtiiiliiii  '"ini 


iS-' 


M 


ir 


work,  he  would  have  found  in  me  an  active  supporter. 
The  trusses  are  without  effective  upper  and  lower 
chords  ;  it  beinjj  originally  supposed  that  the  upper 
nnd  lower  floors  would  act  in  the  sense  of  chords. 
This  has  been  found  to  be  a  mistake  ;  and  the  use- 
fulness of  the  dia<(onals  and  posts  (which  would 
otherwise  answer)  is  impaired  by  the  defect.  I  have 
repeatedly  called  the  attention  of  the  Directors  to 
this  matter,  and  the  fact  of  their  paying  no  attention 
to  my  recommendations  shows  an  almost  unpardon- 
able indifference  on  their  part. 

Passinjjj  on  in  Mr.  Wnsell's  pamphlet,  from  the 
table  of  quantities  given,  we  come  to  certain  assump- 
tions on  which  his  criticism  is  predicated.  These 
are  :  - 

First  Assumption. — That  "each  cable  bears  its 
due  proportion  of  load."  By  which  I  suppose  him 
to  mean,  that  if  two  cables  (under  like  conditions) 
bear  a  certain  load,  one  cable  will  bear  one-half  of 
that  load.  This  is  an  axiom.  The  result  given  under 
such  assumption  would  be  correct,  if  the  /oml  assumed 
had  been  correct.  When  the  proper  load  is  taken, 
the  factor  of  safety  will  be  over  5,  instead  of  3,'*^  as 
he  gives  it.  For  the  test  load  of  326  tons,  the  factor 
was  4j*j5^  for  the  uppt^r  cables,  and  5  for  the  lower. 

Second  Assiimpiton. — That  " each  pair  of  cables, 
whether  upper  or  lower,  carry  their  own  particular 
load."  That  is  to  say,  that  the  t//>/>cr  cables  a/one 
carry  the  railroad  trains  and  receive  the  strain  due  to 
their  entire  weight ;  leaving  the  lower  cables  to  carry 
only  the  light  loads  that  ordinarily  come  on  the  lower 
floor.     This  would  be  perfectly  true  if  the  two  floors 


were  entirely  disconnected  ;  but  as  they  are  united  it 
cannot  be  correct.  This  part  of  the  subject  may  be 
considered  further  under  the  next  head. 

Third  Assumption. — That  "each  pair  of  cables 
carries  its  own  particular  load,  in  addition  to  a  large 
portion  of  the  load  due  to  the  pair  of  cables  above 
or  below  them."  There  is  here  a  kernel  of  truth,  but 
1  shall  proceed  to  prove  that  what  Mr.  Wasell  ex- 
aggerates into  a  mountain,  is  nothing  but  a  mole-hill. 

Considering  the  case  of  the  upper  cable,  we  find 
the  conditions  to  be  approximately  as  shown  in  the 
following  diagram,  viz. :  a  central  half  span  of  410 
feet,  with  54  feet  deflection,  balanced  by  a  land  span 
of  214  feet  (or  half  length  107  feet)  with  a  deflection 
of  nearly  i  foot.* 


The  length  of  the  central  curve  under  these  con- 
ditions is  829yYir  feet,  and  of  the  curve  of  land  span 
^Htoit"!"'  ^  f"'^  ordinary  load  on  each  cable,  in- 
cluding dead  and  live  load,  is  745  pounds  per  foot, 
and  as  the  factor  of  tension  in  this  case  is  lyVirV  ^^^ 
total  tension  resulting  is 

T  =  829TVTr  X  745  X  1^^^=^209,265  pounds. 


*  This  is  on  the  supposition  that  the  land  span  be  considered 
horizontal  instead  of  inclined,  which  does  not  alter  the  effect  of 
expansion,  etc. 


it 


To  balance  this  we  have  the  land  span  weighinj^ 
203  pounds  per  foot,  with  a  factor  of  tension  — 
27.83486,  and 

T=  2 14.01 !  5 1  X  203  X  27.83486  -  1 ,209,267  pounds, 
or  the  tensions  in  the  two  curves  practically  equal. 


r>    rv\r\tf4A    t~s* 


i 


ERRATA. 

The  diagrams  on  pages  8  and  10  should  be  tn>:is- 
posed.     For  the  figures  on  page  9  read  as  foll'Mv.s: — 

The  defleciK  n    f  the  central  curve  then  ==  56.314  feet. 

Th'j  f^ictor  of  tension  in      '         "...  =.-  1.8875      ' 

And  the  total  tension  in     "         "   .     .     .  =  1,166,320  Ihs. 

The  deflection  of  the  hind  span    -     -     .  =  4.256  feet. 

The  factor  of  tension     "        "       .     .     .  =  6.305     " 

And  the  total         .     .     ,  =  273,918  lbs. 

The  excess  of  tension  in  the  centre  span  =  892,402     " 

On  page   12,  in  fifteenth  line  from  top,  for  ^JL-^j^ 

^^^^   1000(7 

motion    towards   the  centre,  at   once   mcreases   the 
deflection,  and  lessens  the  tension  of  the  central  span. 

With  a  .shortening  of  the  cable  (consequent  on  a 
lowering  of  the  temperature)  precisely  the  reverse  of 
the  action  described  takes  place,  and  the  saddles  must 
move  towards  the  land  to  equalize  the  tension.  In 
this  case,  too,  the  motion  will  be  /t-js  than  the  change 
in  length  of  the  land  curve.  If  I  assume,  then,  in  the 
future  discussion,  that  the  .saddles  move  in  or  out  to 


Ml 

'iy 


lO 

the  full  extent  of  the  lengthening  or  shortening  of 
the  land  curve  of  the  cables,  I  am  entirely  on  the  side 
of  safety. 

Mr.  Wasell's  criticism  of  the  bridge  is  based  alto- 
gether upon  his  apparent  proof,  that  when  the  cables 
are  exposed  to  a  low  temperature,  it  is  impossible  that 
the  lower  ones  shall  bear  any  considerable  portion  of 
the  load  on  the  upper  roadway ;  (or,  on  the  contrary, 
that  when  exposed  to  a  high  temperature  the  upper 
cables  will  not  bear  their  proper  load  ;)  and  not  only 
this,  but  that  owing  to  the  rigid  connections  which 
he  claims  to  exist  between  the  cables,  there  will  under 
a  high  temperature  be  a  portion  of  the  weight  of  the 
upper  cables  borne  by  the  lower,  and  under  a  low 
temperature  a  portion  of  the  weight  of  the  lower 
cables  borne  by  the  higher. 

Proceeding  to  actual  facts,  we  find  that  the  cables 
of  the  Niagara  Bridge  were  all  regulated  at  a  mean 
temperature  of  55°  Fah.,*  and  at  that  temperature 
the  deflections,  spans,  and  lengths  of  curves,  were 
as  shown  in  the  following  diagram  : — 


.410 


107. 


20.86118 


*  The  floors  were  also  united  at  that  temperature  ;  and  as  the 
lower  floor  had  been  used  for  over  a  year  ))revious  for  a  carriage-way, 
the  upjier  floor  was  loaded  with  600  tons  of  stone,  to  produce  the 
same  amount  of  stretch  in  the  upper  cables  that  had  already  taken 
place  in  the  lower. 


■'J 


JBH 


dBii 


TI 


From  this  \vc  get : — 

Length  of  upper  cables  each  215.13-1-830.76  +  215.13=1261  feet. 
"  "    lower      "         "      179.24-^834.52-1-179.24=1193     " 

Here  we  strike  at  once,  one  of  the  chief  sources 
of  Mr.  Wasell's  blundering ;  since  he  takes  the  lengths 
of  all  alike  at  "  about  1,460  feet." 

Suppose  now  the  temperature  to  fall  to  20°  below 
zero,  the  changes  in  length  will  then  be  : — 

Land  curve  shortened.     River  curve  shor'ened. 

In  Upper  Cable      -     -     o.iii  feet (M27  fe'et. 

In  Lower     "      -     -     .     0.092     " 0.429     " 

This,  as  previou.sly  shown,  will  cause  each  saddle 
of  the  upper  cables  to  move  towards  the  land  iVjTir 
feet,  and  each  saddle  of  the  lower  cables  to  move 
likewise  -j^'^^i  feet,  thus  increasing  the  spans  to 
821  fVV,  feet  and  82i/L>,yV  feet  respectively.  The 
lengths  of  curves  of  the  river  spans  will  be  shortened 

to  830jWV  f^'^'t  *^"^^  ^34i o¥(T  ^^^^  respectively.  The 
final  effect  of  these  changes  will  be  to  diminish 
the  deflection  of  the  upper  cables  to  52y{|,y  feet,  and 
of  the  lower  cables  to  62  j^jy  feet,  making  the  distance 
between  them  lOfW  feet,  or  43/4  inches  more  than  it 
was  when  first  adjusted. 

Suppose,  on  the  contrary,  the  change  to  have  been 
to  -|-  130°  Fah.,wethen  have  the  cables  elongated  by 
the  same  amount  that  they  were  in  the  other  case 
shortened  ;  and  the  final  effect  will  be  a  deflection  of 
55tVo  f'^'t.'t  "^  ^^^*-'  "PPer  cables  and  6c,^\f^J  feet  in  the 
lower  cables,  or  the  di.stance  between  them  9|"m>  feet. 
This  makes  the  total  difference  in  distances,  between 
the  cables   ,Y,f  feet    --  8^8  inches,  for  these  extreme 


T 


12 


cases.      Mr.  Wascll    only    makes   it   y^   inches,  the 
difference  being  due  to  the  unfavorable  supposition  ' 
we  have  made,  that  the  saddles   move  to  the    full 
amount  that  the  land  spans  elongate.* 

Now  allowing  Mr.  Wasell's  argument  full  weight, 
let  us  see  what  it  amounts  to.  At  the  time  of 
greatest  cold,  we  have  found  that  the  distance 
between  the  cables  is  ^^^  feet  more  than  the  normal 
distance ;  and  it  is  manifest  that  more  load  will  be 
thrown  on  the  upper  cables  in  consequence  of  this 
increase.  The  question,  then,  to  consider  is,  how 
much  will  it  be  necessary  to  ta^c  off  of  the  lower  and 
place  upon  the  upper  cables,  to  bring  them  again  into 
equal  bearing? 

Experiments  show  that  iron  wire  extends  joVo  ^^ 
its  length  per  square  inch  of  section,  for  each  gross 
ton  of  weight  added.  If,  then,  we  add  i8  tons  of 
weight  to  each  of  the  upper  cables,  the  strain  in  each 
will  be  increased  35  jVo  tons.  This  increased  load  will 
cause  the  curves  of  the  land  spans  to  each  increase 
in  length  yW%^  ^QQty  and  the  curve  of  the  centre  span 
to  increase  ^W-^  feet ;  and  the  final  effect  will  be  to 
increase  the  deflection  of  the  centre  span  by  f^j/^j) 
feet,  making  it  52fYA  f*-'<-'t-  But  when  we  place  i8 
tons  more  load  on  each  upper  cable,  we  by  so  doing 
diminish  the  weight  on  the  hnvcr  cables  ;  thus  causing 
a  diminution  of  tension  of  30yVo  tons  in  each.  This 
diminished  strain  will  cause  the  curve  of  each  land 
span  of  the  lower  cables  to  shorten  0.00908  feet,  the 

*  On  the  unallowable  supimsilion  that  the  saddles  are  fixiui  on 
top  of  the  towers,  the  difference  in  tlistances  between  the  cables  for 
the  extreme  range  of  temperatures  taken,  would  be  but  ^%  inches. 


'3 

curve  of  the  middle  span  to  shorten  0.0423  fe'it,  and 
as  a  result  the  centre  of  the  lower  cables  to  rise  y*/^ 
feet,  making  the  total  deflection  62-/^^^  feet.  The 
difference  between  the  total  deflections  of  the  lower 
and  upper  cables  will  then  be  lOjf^  feet,  or  but  one- 
fourth  of  an  inch  in  excess  of  the  mean  dijfcrcnce. 
We  learn,  therefore;  that  the  transfer  of  only  18 
tons  gross  (  =-=  20^'^"^  net  tons)  to  each  upper  cable, 
is  sufficient  to  counteract  the  effect  of  a  temperature 
of  20°  below  zero. 

It  remains  yet  to  be  shown,  what  the  actual 
strain  in  the  upper  cables  will  be  under  these  con- 
ditions;  and  here  I  will  use  the  load  assumed  by  Mr. 
Wasell.     He  places 

The  total  dead  load  at 900  tons. 

"      "      moving  load  at--.- 630      " 

Total 1530 

Load  on  each  of  four  cables  if  all  bear  alike  then  =    382  tons. 
Add  the  load  transferred  to  each  upper  cai)le 

at  20**  below  zero =      20      " 

Total  load  on  each  upper  cable   -     -   ==    402      " 

Total  tension  resulting  from  this  load  will  lie 
402X1.95 =     7S8  tons. 

Ultimate  strength  of  one  cable =  2657.O  " 

Coitsc(juetit  factor  of  safety  «=  — "Vs'—    '"'        3A 

This  is  a  very  different  matter  from  the  low  factor 
given  by  Mr.  Wasell,  viz.,  2-^q,  and  shows  that  even 
when  we  assume  the  monstrous  load  he  has  given,  ive 
find  the  bridge  to  be  safe;  the  total  excess  in  strain 
caused  by  a  change  from  mean  to  extreme  temperature 


I   MIUM   I  Ml 


"'^.'ii  ■ 


?fV« 


•;:'!■:  "'t'lHtlui 


:%r 


rJii... 


i 


H 

either  ivay,  beinir  but  jijf  f.i]  -=  i  >^  per  cent,  of  the  total 
strength  of  the  cabks ;  so  small  does  this  fearful 
bugbear  become  when  critically  examined.* 

In  this  investigation  no  allowance  has  been  made 
for  the  strength  of  the  long  stays.  The  safe  support- 
ing power  of  these  is  150  tons  in  the  aggregate,  or 
about  one-tenth  of  the  load  assumed  by  Mr.  Wasell. 
The  factor  of  safety  of  the  whole  bridge,  therefore, 
under  this  extreme  load,  on  the  supposition  of  equal 
bearing  on  all  the  cables,  would  be  about  4.  I 
admit  that  these  factors  are  smaller  than  is  usual  in 
such  structures,  but  have  already  shown  that  the  true 
factors  under  all  ordinary  loads,  and  even  the  extra- 
ordinary test  load  applied  when  the  bridge  was 
opened,  are  fully  up  to  the  best  engineering  practice 
of  the  day. 

If  the  upper  cables  were  strained  to  their  breaking 
point,  they  would  have  a  deflection  of  65  feet,  or 
eleven  feet  more  than  that  of  their  mean  position. 
Under  the  test  load  of  326  tons,  the  deflection 
was  only  gfs  inches,  returning  to  its  original  amount 
on  the  removal  of  the  load  ;  thus  showing  that  the 
limit  of  elasticity  had   not    been    reached.     In  this 

*  1  am  unwilling  to  allow  that  even  this  small  difterence  in 
strains  actually  occurs.  The  connections  of  the  Ctibles  to  the  floors  is 
not  such  as  to  allow  any  such  action  as  the  author  claims.  When  by 
lowering  the  temperature  there  is  a  tendency  to  an  increase  of  the 
distance  between  the  cables,  the  cradling  of  the  lower  cables  will  be 
increased,  that  is  they  will  swin^f  nearer  together;  and  that  of  the 
upper  cables  will  be  diminished  by  their  separatnig  a  little,  so  that 
finally,  the  vertical  distances  between  the  cables  will  remain  the  same. 
The  reverse  action  will  occur  when  the  temperature  is  raised. 

Avery  casual  inspection  of  the  IJridge  will  make  the  truth  of  this 
statement  manifest  ti>  anv  observer. 


%r 


,(.: 


IJILL. 


^kO rid . 

'5 

connection  it  is  proper  to  state,  that  the  material 
in  a  suspension  bridge  is  in  the  best  possible  state 
(being  always  in  tension)  to  bear  its  load ;  and  can 
safely  be  loaded  to  much  nearer  its  full  breaking 
strength  than  where  exposed,  as  in  many  other 
structures,  to  strains  first  of  tension,  and  then  of 
compression.  Experiments  prove  this  beyond  all 
question  ;  and  it  is  an  element  of  economy  for  which 
Mr.  Wasell  gives  no  credit. 

I  have  thus  far  shown  that  the  writer  has  fallen 
into  error,  first,  in  his  assumption  of  load  ;  and  second, 
as  to  the  length  of  the  cables.  Thirdly,  that  by 
making  no  allowance  for  the  self-adjustment  of  the 
system  (by  an  increase  or  diminution  of  the  cradling 
of  the  cables)  he  induces  a  condition  of  things  which 
cannot  possibly  exist ;  and  finally,  that  even  on  the 
supposition  of  fixity  of  position,  the  elasticity  of  the 
material  in  the  cables  allows  of  but  an  insignificant 
increase  of  strain  in  either  pair  of  cables  before  the 
other  comes  into  full  bearing ;  the  final  conclusion 
being,  that  even  under  the  most  extreme  conditions 
of  temperature  and  load,  ?io  danger  is  to  he  appre- 
hended to  the  structure. 

1  propose,  in  closing,  now  that  I  have  disproved 
the  defects  claimed  by  Mr.  Wasell,  to  show  that  the 
dc  i';n  for  the  Niagara  bridge  has  some  positive 
adv:  r.tagcs,  which  are  derived  from  what  he  considers 
a  faulty  construction. 

It  would  have  been  very  easy  to  have  arranged 
the  land  cables  so  as  to  compensate  exactly  for  the 
differences  in  deflections  in  the  main  span ;  but  the 
upper  ones  were  made  longer  because  one  anchorage 


'W...:M 


'.f 


r 


i6 


had  to  come  behind  the  other  (to  accommodate  other 
more  important  structural  conditions),  thus  requiring 
a  certain  space  independently  of  other  considerations. 
It  was  necessary  to  give  the  cables  such  relative 
positions  that  the  final  resultant  of  the  various  lines 
of  pressure  in  the  towers  should  intersect  their  bases 
as  near  the  centre  as  possible.  By  tracing  the  various 
planes  in  which  the  several  cables  lie,  it  will  be  found 
that  this  result  is  reached  by  the  plan  adopted  as  near 
as  may  be.  It  was  this  ingenious  arrangement  that 
permitted  the  use  of  those  slender  obelisks  which 
now  form  the  towers  of  the  bridge  ;  whereas  any 
deviation  would  have  necessitated  double  the  amount 
of  masonry,  besides  a  connecting  arch  at  the  summit, 
coupled  with  a  threefold  expenditure  of  money. 

A  second  advantage  of  the  design  is,  that  the 
cables  are  attached  to  the  superstructure  with  its 
moving  loads,  in  such  a  way  as  to  give  the  greatest 
amount  of  stability.  The  centre  of  gravity  is  low 
enough  to  avoid  all  top-heaviness  due  to  the  load 
on  the  railroad  floor.  With  a  higher  centre  of 
gravity,  lateral  oscillations  would  occur,  and  the 
chances  of  de-railment  would  be  very  great.  There 
would  have  been  no  difficulty  in  suspending  both 
sets  of  cables  at  the  same  deflection,  either  on 
the  level  of  the  railroad  or  cff  the  lower  floor,  at  a 
sacrifice,  however,  of  the  advantages  alre?idy  named. 
If  suspended  at  the  railroad  level,  the  pendent 
truss  and  lower  floor  would  oscillate  from  side  to  side 
under  the  influence  of  the  wind,  besides  increasing 
the  aggregate  tension  in  both  cables,  which  would 
have  to  be  met  by  higher  towers  or  hepvier  cables. 
On  the  other  hand,  to  suspend  them  at  the  lower 


1 


*i 


r,'~'iiij'ti  (ti> 


' 


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ISBfl 


iMSiiaM! 


img^ 


1/ 

level  would  be  the  heij^lit  of  folly,  as  the  hij^h  centre 
of  gravity  would  destroy  all  st^nbility.  The  relative 
grades  of  railroad  and  common  road  were  also  such 
as  to  enforce  the  use  of  the  upper  floor  by  the 
railroad. 

A  third  advantage  of  the  design  consists  in  this : 
that  cables  of  the  same  span  but  different  deflections 
oscillate  in  different  times,  and  with  different  wave 
lengths;  hence,  when  a  pair  of  such  cables  are 
attached  to  a  structure,  the  oscillations  caused  in 
one  by  a  load  or  the  wind,  tend  constantly  to  neu- 
tralize those  caused  in  the  other,  and  to  produce  a 
state  of  rest.  In  conclusion,  I  quite  agree  with  Mr. 
Wasell  that  "  the  suppression  of  facts  so  intelligible, 
and  of  such  vital  importance  to  the  travelling  public 
would  [indeed I  be  criminal";  but  the  facts,  on  ex- 
amination, have  dwindled  to  such  diminutive  pro- 
portions that,  in  my  judgment,  and  I  trust  that  of 
the  public,  they  do  not  yet  justify  the  building  of 
Mr.  Wasell's  "  Improved  Trussed  Girder  Bridge," 
either  at  the  Falls  or  any  other  point  on  the 
Niagara  River. 


i 


VV.  A.  ROEBLING. 


New  York,  Nov.  28///,  1876. 


